Regular expressions (regexps) are patterns which describe the
contents of a string. They’re used for testing whether a string contains a
given pattern, or extracting the portions that match. They are created with
the /pat/ and
%r{pat} literals or the
Regexp.new constructor.

A regexp is usually delimited with forward slashes (/). For
example:

/hay/=~'haystack'#=> 0/y/.match('haystack') #=> #<MatchData "y">

If a string contains the pattern it is said to match. A literal
string matches itself.

# 'haystack' does not contain the pattern 'needle', so doesn't match./needle/.match('haystack') #=> nil# 'haystack' does contain the pattern 'hay', so it matches/hay/.match('haystack') #=> #<MatchData "hay">

Specifically, /st/ requires that the string contains the
letter s followed by the letter t, so it matches
haystack, also.

The following are metacharacters(, ),
[, ], {, },
., ?, +, *. They have a
specific meaning when appearing in a pattern. To match them literally they
must be backslash-escaped. To match a backslash literally backslash-escape
that: \\\.

/1 \+ 2 = 3\?/.match('Does 1 + 2 = 3?') #=> #<MatchData "1 + 2 = 3?">

Patterns behave like double-quoted strings so can contain the same
backslash escapes.

/\s\u{6771 4eac 90fd}/.match("Go to 東京都")
#=> #<MatchData " 東京都">

Arbitrary Ruby expressions can be embedded into patterns with the
#{...} construct.

A character class is delimited with square brackets
([, ]) and lists characters that may appear at
that point in the match. /[ab]/ means a or
b, as opposed to /ab/ which means a followed
by b.

/W[aeiou]rd/.match("Word") #=> #<MatchData "Word">

Within a character class the hyphen (-) is a metacharacter
denoting an inclusive range of characters. [abcd] is
equivalent to [a-d]. A range can be followed by another range,
so [abcdwxyz] is equivalent to [a-dw-z]. The
order in which ranges or individual characters appear inside a character
class is irrelevant.

If the first character of a character class is a caret (^) the
class is inverted: it matches any character except those named.

/[^a-eg-z]/.match('f') #=> #<MatchData "f">

A character class may contain another character class. By itself this isn’t
useful because [a-z[0-9]] describes the same set as
[a-z0-9]. However, character classes also support the
&& operator which performs set intersection on its
arguments. The two can be combined as follows:

/[a-w&&[^c-g]z]/# ([a-w] AND ([^c-g] OR z))# This is equivalent to:/[abh-w]/

The following metacharacters also behave like character classes:

/./ - Any character except a newline.

/./m - Any character (the m modifier enables
multiline mode)

/\w/ - A word character ([a-zA-Z0-9_])

/\W/ - A non-word character ([^a-zA-Z0-9_])

/\d/ - A digit character ([0-9])

/\D/ - A non-digit character ([^0-9])

/\h/ - A hexdigit character ([0-9a-fA-F])

/\H/ - A non-hexdigit character ([^0-9a-fA-F])

/\s/ - A whitespace character: /[ \t\r\n\f]/

/\S/ - A non-whitespace character: /[^ \t\r\n\f]/

POSIX bracket expressions are also similar to character classes.
They provide a portable alternative to the above, with the added benefit
that they encompass non-ASCII characters. For instance, /\d/
matches only the ASCII decimal digits (0-9); whereas
/[[:digit:]]/ matches any character in the Unicode Nd
category.

/[[:alnum:]]/ - Alphabetic and numeric character

/[[:alpha:]]/ - Alphabetic character

/[[:blank:]]/ - Space or tab

/[[:cntrl:]]/ - Control character

/[[:digit:]]/ - Digit

/[[:graph:]]/ - Non-blank character (excludes spaces, control
characters, and similar)

/[[:lower:]]/ - Lowercase alphabetical character

/[[:print:]]/ - Like [:graph:], but includes the space
character

/[[:punct:]]/ - Punctuation character

/[[:space:]]/ - Whitespace character ([:blank:],
newline,

carriage return, etc.)

/[[:upper:]]/ - Uppercase alphabetical

/[[:xdigit:]]/ - Digit allowed in a hexadecimal number (i.e.,
0-9a-fA-F)

Ruby also supports the following non-POSIX character classes:

/[[:word:]]/ - A character in one of the following Unicode
general categories Letter, Mark, Number,
Connector_Punctuation

The constructs described so far match a single character. They can be
followed by a repetition metacharacter to specify how many times they need
to occur. Such metacharacters are called quantifiers.

* - Zero or more times

+ - One or more times

? - Zero or one times (optional)

{n} - Exactly n times

{n,} - n or more times

{,m} - m or less times

{n,m} - At least
n and at most m times

# At least one uppercase character ('H'), at least one lowercase# character ('e'), two 'l' characters, then one 'o'"Hello".match(/[[:upper:]]+[[:lower:]]+l{2}o/) #=> #<MatchData "Hello">

Repetition is greedy by default: as many occurrences as possible
are matched while still allowing the overall match to succeed. By contrast,
lazy matching makes the minimal amount of matches necessary for
overall success. A greedy metacharacter can be made lazy by following it
with ?.

# Both patterns below match the string. The first uses a greedy# quantifier so '.+' matches '<a><b>'; the second uses a lazy# quantifier so '.+?' matches '<a>'./<.+>/.match("<a><b>") #=> #<MatchData "<a><b>">/<.+?>/.match("<a><b>") #=> #<MatchData "<a>">

A quantifier followed by + matches possessively: once
it has matched it does not backtrack. They behave like greedy quantifiers,
but having matched they refuse to “give up” their match even if this
jeopardises the overall match.

Parentheses can be used for capturing. The text enclosed by the
n<sup>th</sup> group of parentheses can be
subsequently referred to with n. Within a pattern use the
backreference</tt>n; outside of the pattern
use <tt>MatchData[n].

# 'at' is captured by the first group of parentheses, then referred to# later with \1/[csh](..) [csh]\1 in/.match("The cat sat in the hat")
#=> #<MatchData "cat sat in" 1:"at"># Regexp#match returns a MatchData object which makes the captured# text available with its #[] method./[csh](..) [csh]\1 in/.match("The cat sat in the hat")[1] #=> 'at'

Capture groups can be referred to by name when defined with the
(?<name>) or
(?'name') constructs.

The (?:…) construct provides grouping without
capturing. That is, it combines the terms it contains into an atomic whole
without creating a backreference. This benefits performance at the slight
expense of readabilty.

# The group of parentheses captures 'n' and the second 'ti'. The# second group is referred to later with the backreference \2/I(n)ves(ti)ga\2ons/.match("Investigations")
#=> #<MatchData "Investigations" 1:"n" 2:"ti"># The first group of parentheses is now made non-capturing with '?:',# so it still matches 'n', but doesn't create the backreference. Thus,# the backreference \1 now refers to 'ti'./I(?:n)ves(ti)ga\1ons/.match("Investigations")
#=> #<MatchData "Investigations" 1:"ti">

Grouping can be made atomic with
(?>pat). This causes the
subexpression pat to be matched independently of the rest of the
expression such that what it matches becomes fixed for the remainder of the
match, unless the entire subexpression must be abandoned and subsequently
revisited. In this way pat is treated as a non-divisible whole.
Atomic grouping is typically used to optimise patterns so as to prevent the
regular expression engine from backtracking needlesly.

# The <tt>"</tt> in the pattern below matches the first character of# the string, then <tt>.*</tt> matches <i>Quote"</i>. This causes the# overall match to fail, so the text matched by <tt>.*</tt> is# backtracked by one position, which leaves the final character of the# string available to match <tt>"</tt>/".*"/.match('"Quote"') #=> #<MatchData "\"Quote\""># If <tt>.*</tt> is grouped atomically, it refuses to backtrack# <i>Quote"</i>, even though this means that the overall match fails/"(?>.*)"/.match('"Quote"') #=> nil

The \g<name> syntax matches the
previous subexpression named name, which can be a group name or
number, again. This differs from backreferences in that it re-executes the
group rather than simply trying to re-match the same text.

# Matches a <i>(</i> character and assigns it to the <tt>paren</tt># group, tries to call that the <tt>paren</tt> sub-expression again# but fails, then matches a literal <i>)</i>./\A(?<paren>\(\g<paren>*\))*\z/=~'()'/\A(?<paren>\(\g<paren>*\))*\z/=~'(())'#=> 0# ^1# ^2# ^3# ^4# ^5# ^6# ^7# ^8# ^9# ^10

Matches at the beginning of the string, i.e. before the first character.

Enters a named capture group called paren

Matches a literal (, the first character in the string

Calls the paren group again, i.e. recurses back to the second
step

Re-enters the paren group

Matches a literal (, the second character in the string

Try to call paren a third time, but fail because doing so
would prevent an overall successful match

Match a literal ), the third character in the string. Marks the
end of the second recursive call

(?=pat) - Positive lookahead
assertion: ensures that the following characters match pat, but
doesn't include those characters in the matched text

(?!pat) - Negative lookahead
assertion: ensures that the following characters do not match pat,
but doesn't include those characters in the matched text

(?<=pat) - Positive
lookbehind assertion: ensures that the preceding characters match
pat, but doesn't include those characters in the matched text

(?<!pat) - Negative
lookbehind assertion: ensures that the preceding characters do not
match pat, but doesn't include those characters in the matched
text

# If a pattern isn't anchored it can begin at any point in the string
/real/.match("surrealist") #=> #<MatchData "real">
# Anchoring the pattern to the beginning of the string forces the
# match to start there. 'real' doesn't occur at the beginning of the
# string, so now the match fails
/\Areal/.match("surrealist") #=> nil
# The match below fails because although 'Demand' contains 'and', the
pattern does not occur at a word boundary.
/\band/.match("Demand")
# Whereas in the following example 'and' has been anchored to a
# non-word boundary so instead of matching the first 'and' it matches
# from the fourth letter of 'demand' instead
/\Band.+/.match("Supply and demand curve") #=> #<MatchData "and curve">
# The pattern below uses positive lookahead and positive lookbehind to
# match text appearing in <b></b> tags without including the tags in the
# match
/(?<=<b>)\w+(?=<\/b>)/.match("Fortune favours the <b>bold</b>")
#=> #<MatchData "bold">

As mentioned above, the x option enables free-spacing
mode. Literal white space inside the pattern is ignored, and the octothorpe
(#) character introduces a comment until the end of the line.
This allows the components of the pattern to be organised in a potentially
more readable fashion.

# A contrived pattern to match a number with optional decimal placesfloat_pat = /\A
[[:digit:]]+ # 1 or more digits before the decimal point
(\. # Decimal point
[[:digit:]]+ # 1 or more digits after the decimal point
)? # The decimal point and following digits are optional
\Z/xfloat_pat.match('3.14') #=> #<MatchData "3.14" 1:".14">

Note: To match whitespace in an x pattern use
an escape such as \s or \p{Space}.

Comments can be included in a non-x pattern with the
(?#comment) construct, where
comment is arbitrary text ignored by the regexp engine.

Regular expressions are assumed to use the source encoding. This can be
overridden with one of the following modifiers.

/pat/u - UTF-8

/pat/e - EUC-JP

/pat/s - Windows-31J

/pat/n - ASCII-8BIT

A regexp can be matched against a string when they either share an
encoding, or the regexp’s encoding is US-ASCII and the string’s
encoding is ASCII-compatible.

If a match between incompatible encodings is attempted an
Encoding::CompatibilityError exception is raised.

The Regexp#fixed_encoding? predicate indicates whether the
regexp has a fixed encoding, that is one incompatible with ASCII.
A regexp’s encoding can be explicitly fixed by supplying
Regexp::FIXEDENCODING as the second argument of
Regexp.new:

Certain pathological combinations of constructs can lead to abysmally bad
performance.

Consider a string of 25 as, a d, 4 as, and a
c.

s = 'a'*25+'d''a'*4+'c'#=> "aaaaaaaaaaaaaaaaaaaaaaaaadadadadac"

The following patterns match instantly as you would expect:

/(b|a)/ =~ s #=> 0
/(b|a+)/ =~ s #=> 0
/(b|a+)*\/ =~ s #=> 0

However, the following pattern takes appreciably longer:

/(b|a+)*c/=~s#=> 32

This happens because an atom in the regexp is quantified by both an
immediate + and an enclosing * with nothing to
differentiate which is in control of any particular character. The
nondeterminism that results produces super-linear performance. (Consult
Mastering Regular Expressions (3rd ed.), pp 222, by Jeffery
Friedl, for an in-depth analysis). This particular case can be fixed
by use of atomic grouping, which prevents the unnecessary backtracking:

A similar case is typified by the following example, which takes
approximately 60 seconds to execute for me:

# Match a string of 29 <i>a</i>s against a pattern of 29 optional# <i>a</i>s followed by 29 mandatory <i>a</i>s.Regexp.new('a?'*29+'a'*29) =~'a'*29

The 29 optional as match the string, but this prevents the 29
mandatory as that follow from matching. Ruby must then backtrack
repeatedly so as to satisfy as many of the optional matches as it can while
still matching the mandatory 29. It is plain to us that none of the
optional matches can succeed, but this fact unfortunately eludes Ruby.

One approach for improving performance is to anchor the match to the
beginning of the string, thus significantly reducing the amount of
backtracking needed.